As mobile devices have been increasingly developed, and the demand for such mobile devices has increased, the demand for secondary batteries has also sharply increased as an energy source for the mobile devices. Among such secondary batteries is a lithium secondary battery having a high energy density, a high operation potential, a long cycle lifespan, and a low self discharge rate, which is now commercialized and widely used.
In addition, in recent years, with increased concerns about environmental problems, much research has been carried out into electric vehicles (EV) and hybrid electric vehicles (HEV), which are capable of substituting for vehicles using fossil fuel, such as gasoline and diesel oil, which cause air pollution. Nickel-metal hydride (Ni-MH) secondary batteries have been mainly used as a power source for the electric vehicles (EV) and the hybrid electric vehicles (HEV). On the other hand, much research has also been carried out into lithium secondary batteries having a high energy density, high discharge voltage, and high output stability, and some of the lithium secondary batteries are now commercialized.
A lithium secondary battery is configured to have a structure in which an electrode assembly, including a positive electrode and a negative electrode each having an active material applied to a current collector and a porous separator interposed between the positive electrode and the negative electrode, is impregnated with a non-aqueous electrolyte containing lithium salt.
A lithium cobalt oxide, a lithium manganese oxide, a lithium nickel oxide, or a lithium composite oxide is used as the positive electrode active material of the lithium secondary battery. Carbon is mainly used as the negative electrode active material of the lithium secondary battery. The use of a silicon compound or a sulfate compound as the negative electrode active material of the lithium secondary battery is also under consideration.
At the time of manufacturing a battery for vehicles requiring high power characteristics, it is increasingly necessary to use a positive electrode material that is capable of improving power in a low voltage range. Lithium iron phosphate (LiFePO4) has a lower voltage range than a conventional positive electrode active material, such as a ternary material (LiNiMnCoO2) or spinel manganese (LiMn2O4), which has been widely used.
For high power, amorphous carbon (hard carbon, soft carbon, etc.) has been used as the negative electrode active material of the secondary battery. For the amorphous carbon, however, operating voltage of a discharge terminal is high. In a case in which lithium iron phosphate is used as the positive electrode active material of the secondary battery, therefore, a power increase effect is not satisfied.
Therefore, there is a high necessity for technology that is capable of fundamentally solving the above problems.